There are many examples of utilizing light through a optical waveguide to sense the change in refractive index of a liquid as an indication of the change in another characteristic of the liquid. For example:
U.S. Pat. No. 4,050,895, Sep. 27, 1977, of E. Hardy et al. discloses a cylindrical waveguide that is coated with a second material that reacts with the first material to be sensed. The reaction causes a change in color or other characteristic of the coating that effects the amount of light transmitted through the waveguide.
U.S. Pat. No. 4,187,025, Feb. 5, 1980, of A. Harmer discloses a short length of optical guideway having multiple bends that is immersed in a liquid under test. As shown in FIG. 6 of the xe2x80x2025 patent, a small change in the index of refraction of battery acid caused by a change in state of charge of the battery show a detectable change in contrast coefficient xcex93 for the multiple bend embodiments of FIGS. 3 and 4, and no change in contrast for the single bend structures of FIGS. 1 and 2.
U.S. Pat. No. 4,427,293, Jan. 24, 1984, of Harmer discloses a comparative system for compensating for the effect of temperature on the refractive index of liquids (typically, liquids have a temperature coefficient on the order of xe2x88x925xc3x9710xe2x88x924 /xc2x0 C.).
J. Weiss, Piezooptic behavior of certain fluids, Applied Optics, Vol. 24, No. 8, 15 April 1985, pp. 1151-1155, shows in FIG. 9 curve 1(e) of this invention. This reference teaches that the refractive index of sensor fibers should be close the index of the liquid.
U.S. Pat. No. 4,851,817, Jul. 25, 1989, of C. Brossia et al. discloses a system for detecting ice or water on a surface with a short length of relatively straight optical waveguide with an external surface that has been abraded perpendicular to its axis. The device detects a relatively large change in index of refraction from 1 (for air) to 1.3 (for ice).
U.S. Pat. No. 5,141,310, Aug. 25, 1992, of A. Boiarski discloses a system for measuring the index of refraction of urine that has a multiple bend waveguide having a core refractive index that is greater than the index of high specific gravity urine (about 1.37), but not so great as to detract from accuracy and sensitivity. A preferred range is stated to be between 1.4 and 1.5.
It is an object of this invention to detect a small change in index of refraction of a liquid using a simple optical waveguide.
It is another object of this invention to detect a small change in index of refraction of a liquid by measuring the change in light through an optical fiber.
Additional objects, advantages, and novel features of the invention will become apparent to those skilled in the art upon examination of the following description or may be learned by practice of the invention.
To achieve the foregoing and other objects, and in accordance with the purpose of the present invention, as embodied and broadly described herein, the present invention may comprise a short straight length of optical guide having a smooth exterior surface along its length covered by a liquid, and a device for inputting a light beam into said guide. The angle of the light beam with respect to the optical guide and the index of refraction ng of the guide core is selected so that light impinges on the interface between the exterior surface and the liquid at the lowest critical angle of the guide. The critical angle changes as the index of the liquid changes such that after the change there is a detectable shift in the amount of light passing through the guide.